This application is based on Japanese Patent Application No. 2002-271305 filed on Sep. 18, 2002, the disclosure of which is incorporated herein by reference.
The present invention relates to a vehicle air conditioner having a variable displacement compressor. More particularly, the present invention relates to a discharge capacity control of the variable displacement compressor.
In a control device of a variable displacement compressor described JP-A-10-278567, a target discharge pressure Pdxe2x80x2 of the variable displacement compressor is calculated based on an engine rotational speed Ne and an inside air temperature Tr of a passenger compartment, in order to reduce consumption power. Further, an actual pressure Pd of refrigerant discharged from the variable displacement compressor is controlled using the target discharge pressure Pdxe2x80x2.
On the other hand, in a variable displacement compressor device described in JP-A-5-99156, a driving voltage Vn applied to an idling adjustment valve is calculated in accordance with the following formula (1).
Vn=Vnxe2x88x921+Kp(Enxe2x88x92Enxe2x88x921)+(Q/Ti)Enxe2x80x83xe2x80x83(1)
wherein, Kp, Q and Ti are fixed control constants that are set beforehand, En is a deviation between a target rotation speed of the compressor and an actual detected rotation speed of the compressor at the process cycle of xe2x80x9cnxe2x80x9d, and Enxe2x88x921 is a deviation between a target rotation speed of the compressor and an actual detected rotation speed of the compressor at the process cycle of xe2x80x9cnxe2x88x921xe2x80x9d. However, in this calculation formula (1), the fixed control constants are required to be set suitably around a middle displacement area of the variable displacement compressor. Accordingly, when the variable displacement compressor operates by a small displacement in a case where a thermal load of an air conditioner is small, the temperature of air discharged from an evaporator is readily changed. Therefore, the temperature of air blown into the passenger compartment is difficult to be accurately controlled.
In view of the above-described problems, it is an object of the present invention to provide a vehicle air conditioner that stably controls a variable displacement compressor in a wide range of thermal load while reducing variation in temperature of air blown from an evaporator.
According to the present invention, in a vehicle air conditioner, a refrigerant cycle includes a variable displacement compressor for compressing refrigerant, a condenser for cooling refrigerant from the variable displacement compressor, a decompression unit for decompressing refrigerant flowing from the condenser, and an evaporator that is disposed in an air conditioning duct to cool air by evaporating refrigerant after being decompressed in the decompression unit. The air conditioner has a load detecting unit for detecting a thermal load in the refrigerant cycle (e.g., evaporator), and a control unit for controlling a discharge capacity of the variable displacement compressor. The variable displacement compressor includes a control valve that continuously changes the discharge capacity of the variable displacement compressor. In the vehicle air conditioner, the control unit calculates an electrical value to be applied to the control valve based on a predetermined calculation formula having at least a control constant, and supplies the electrical value to the control valve so as to control the discharge capacity of the variable displacement compressor. In addition, the control unit changes the control constant in the predetermined calculation formula in accordance with the thermal load. Accordingly, it is possible to stably control the operation of the refrigerant cycle in a wide thermal load range between a high load area and a low load area. Therefore, the variation in air temperature at the air outlet of the evaporator can be made smaller.
Preferably, the load detecting unit includes refrigerant state detecting means for detecting one of a pressure and a flow amount of high-pressure refrigerant before being decompressed in the decompression unit, and the control unit changes the control constant in the predetermined calculation formula, in accordance with the one of the pressure and the flow amount of the high-pressure refrigerant. Further, the control unit calculates a target evaporator air temperature based on the outside air temperature, and calculates the electrical value by using a difference between the evaporator air temperature detected by the evaporator temperature detecting means and the target evaporator air temperature. Therefore, the air temperature at the air outlet of the evaporator can quickly approach to the target evaporator air temperature. For example, the thermal load is at least one of an air temperature at an air outlet of the evaporator, a temperature difference between a target air temperature to be blown into the passenger compartment and an inside temperature of the passenger compartment, a correction value of the air temperature at the air outlet of the evaporator, which is corrected by the outside air temperature, and a correction value of the temperature difference between the target air temperature and the inside temperature, which is corrected by the outside air temperature.
Specifically, the predetermined calculation formula is DT=DT(nxe2x88x921)+Kp[(Enxe2x88x92Enxe2x88x921)+C/(Tixc3x97En)]. In this formula, En=Texe2x88x92Teo, Kp=Kpxe2x80x2/(Phxc3x97Phxe2x80x2), Ti=Tixe2x80x2/(Phxc3x97Phxe2x80x3), C is a sampling cycle, n is a positive number, Te is the detected air temperature at the air outlet of the evaporator, Teo is the evaporator target air temperature, Ph is the refrigerant pressure at the high pressure side, and Kpxe2x80x2, Tixe2x80x2, Phxe2x80x2 and Phxe2x80x3 are constants. Therefore, the variable displacement compressor can be stably controlled in the wide thermal-load range, while the variation in the air temperature at the air outlet of the evaporator can be made small.